Apparatus and method for computing a logarithm of a floating-point number

ABSTRACT

An apparatus for computing a logarithm to a base p of a floating-point number X. The floating-point number X is represented in the format of (−1) Sx ·2 Ex ·M x , where M x =(1+f x )=(1+A x ·2 −K )+(B x ·2 −N ), where S x  is a sign, E x  is an exponent, M x  is a mantissa, 1≦M x &lt;2, f x  is a N-bit fraction, A x  is a value of the most significant K bits of f x , B x  is a value of the least significant (N−K) bits of f x , 0≦K&lt;N, and p, K, N are natural numbers. The apparatus includes: a first multiplier, a logarithmic table, a first adder, a divider, a Taylor-Series approximation circuit, a second multiplier, and a second adder.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates in general to an apparatus and a method to perform logarithmic computations, and more particularly to the precise computation of a logarithm to a base p of a floating-point number.

[0003] 2. Description of the Related Art

[0004] The representation of floating-point numbers is similar to commonly used scientific notation and consists of two parts, the mantissa M and the exponent. The floating-point number F represented by the pair (M, E) has the value,

F=M×β ^(E)

[0005] Where β is the base of the exponent.

[0006] In an effort to unify methods employed in computer systems for performing binary floating-point arithmetic, the IEEE in the early 1980s standardized computer floating-point numbers. Such binary floating-point numbers make possible the manipulation of large as well as small numbers with great precision, and thus are often used in scientific calculations. They typically comprise either single precision format or double precision format, with single precision operating on 32-bit operands and double precision operating on 64-bit operands. Both single and double precision numbers constitute a bit-string characterized by three fields: a single sign bit, several exponent bits, and several fraction or mantissa bits, with the sign bit being the most significant bit, the exponent bits being the next most significant, and the mantissa bits being the least significant.

[0007]FIG. 1 is a diagram showing the form of the single format. Hence, base 2 was selected, a flowing point number F in the single format has the form:

F=(−1)^(S)·2^(E−127)·(1.f)

[0008] where S=sign bit;

[0009] E=8-bit exponent biased by 127;

[0010] f=F's 23-bit fraction or mantissa which, together an implicit leading 1, yield the significant digit field “1. - - - ”.

[0011] Presently, the calculation for the floating-point is used for all kinds of calculations. Computing efficiency depends on efficiency of the calculation of the floating-point. For logarithmic computation of a floating-point number, a logarithmic table is usually determined in advance. Then, the result is found by checking the table. However, when using the logarithmic table, there is a problem of precision. An 8-bit logarithmic table is quite large, but if an 8-bit logarithmic table is used for logarithmic computation of a flouting-point number, the precision of the computing result is not enough. Because the mantissa part of the flouting-point number has 23 bits, to precisely compute a logarithmic of a floating-point number, it is not enough to check an 8-bit logarithmic table.

SUMMARY OF THE INVENTION

[0012] In view of the above, an object of the present invention is to provide an apparatus and a method for precisely computing a logarithm to a base p of a floating-point number.

[0013] For the purpose of the present invention, the invention provides an apparatus for computing a logarithm to a base p of a floating-point number X wherein the floating-point number X is represented in the format of (−1)^(Sx)·2^(Ex)·M_(x), where M_(x)=(1+f_(x))=(1+A_(x)·2^(−K))+(B_(x)·2^(−N)), where S_(x) is a sign, E_(x) is an exponent, M_(x) is a mantissa, 1≦M_(x)<2, f_(x) is a N-bit fraction, A_(x) is a value of the most significant K bits of f_(x), B_(x) is a value of the least significant (N−K) bits of f_(x), 0≦K<N, and p, K, N are natural numbers. The apparatus comprises: a first multiplier for multiplying a number whose value is log_(p)2 and the exponent E_(x) and outputting a multiplying result; a logarithmic table for receiving the value A_(x) and checking the logarithmic table to output a result; a first adder connected to the first multiplier and the logarithmic table for adding the multiplying result and the result to output an adding result; a divider for receiving the value B_(x) and an adding number whose value is (2^(K)+A_(x)) and dividing the value B_(x) by the adding number to output a dividing result R_(d); a Taylor-Series approximation circuit connected to the divider for receiving the dividing result R_(d), finding a value of ln(1+R_(d)) and outputting the value of ln(1+R_(d)); a second multiplier for multiplying a number whose value is 1/ln(p) and the value of ln(1+R_(d)) to output a second multiplying result; and a second adder connected to the first adder and the second multiplier for adding the adding result and the second multiplying result to output the logarithm Y.

[0014] Furthermore, the invention provides a method for computing a logarithm to a base p of a floating-point number X wherein the floating-point number X is represented in the format of (−1)^(Sx)·2^(Ex)·M_(x), where M_(x)=(1+f_(x))=(1+A_(x)·2^(−K))+(B_(x)·2^(−N)), where S_(x) is a sign, E_(x) is an exponent, M_(x) is a mantissa, 1≦M_(x)<2, f_(x) is a N-bit fraction, A_(x) is a value of the most significant K bits of f_(x), B_(x) is a value of the least significant (N−K) bits of f_(x), 0≦K<N, and p, K, N are natural numbers. The method comprises the steps of: multiplying a number whose value is log_(p) 2 and the exponent E_(x) to get a multiplying result to output by a first multiplier; receiving the value A_(x) by a logarithmic table and checking the logarithmic table to get a result to output; adding the multiplying result and the result to get an adding result to output by a first adder; dividing the value B_(x) by an adding number whose value is (2^(K)+A_(x)) to get a dividing result R_(d) to output by a divider; receiving the dividing result R_(d) and finding a value of ln(1+R_(d)) to output by a Taylor-Series approximation circuit; multiplying a number whose value is 1/ln(p) and the exponent E_(x) to get a second multiplying result to output by a second multiplier; adding the adding result and the second multiplying result to get the logarithm Y to output by a second adder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:

[0016]FIG. 1 is a diagram showing the form of the single format;

[0017]FIG. 2 is a schematic diagram showing the apparatus for computing a logarithm of a floating-point number according to the present invention;

[0018]FIG. 3 is a schematic diagram showing the apparatus for computing a logarithm of a floating-point number according to the first embodiment of the present invention;

[0019]FIG. 4 is a schematic diagram showing one example of the apparatus for computing a Taylor-Series approximation circuit according to the first embodiment of FIG. 3;

[0020]FIG. 5 is a schematic diagram showing the apparatus for computing a logarithm of a floating-point number according to the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021]FIG. 2 is a schematic diagram showing the apparatus for computing a logarithm of a floating-point number according to the present invention. The apparatus 100, as shown in FIG. 2, comprises a first multiplier 110, a logarithmic table 120, a first adder 130, a divider 140, a Taylor-Series Approximation Circuit 150, a second adder 160 and a second multiplier 170.

[0022] The apparatus 100 is used to compute a logarithm Y to a base p of a floating-point number X wherein the floating-point number X is represented in the format of (−1)^(Sx)·2^(Ex)·M_(x), where M_(x)=(1+f_(x))=(1+A_(x)·2^(−K))+(B_(x)·2^(−N)), where S_(x) is a sign, E_(x) is an exponent, M_(x) is a mantissa, 1≦M_(x)<2, f_(x) is a N-bit fraction, A_(x) is a value of the most significant K bits of f_(x), B_(x) is a value of the least significant (N−K) bits of f_(x), 0≦K<N, and K, N are natural numbers. The first multiplier 110 receives a number whose vaule is log_(p) 2 and the exponent E_(x) and outputs a multiplying result R_(m1). The logarithmic table 120 receives the value A_(x) and checking the logarithmic table 120 to output a result of log_(p) (1+A_(x)·2^(−K)). The first adder 130 is connected to the first multiplier 110 and the logarithmic table 120 for adding the multiplying result R_(m1) and the result of log_(p) (1+A_(x)·2^(−K)) to output an adding result R_(a). The divider 140 receives the value B_(x) and an adding number whose value is (2^(K)+A_(x)), and divides the value B_(x) by the adding nubmer to output a dividing result R_(d). The Taylor-Series approximation circuit 150 is connected to the divider 140 for receiving the dividing result R_(d), finding a value of ln(1+R_(d)) and outputting the value of ln(1+R_(d)). The second multiplier 170 receives a number whose value is 1/ln(P) and the value of ln(1+R_(d)) to output a second multiplying result R_(m2). The second adder 160 connected to the first adder 150 and the second multiplier 170 for adding the adding result R_(a) and the second multiplying result R_(m2) to output the logarithm Y.

[0023] The floating-point number X is represented in the following format:

X=(−1)^(Sx)·2^(Ex)·M_(x)  (1)

[0024] where M_(x)=(1+f_(x))=(1+A_(x)·2^(−K))+(B_(x)·2^(−N)), where S_(x) is a sign, E_(x) is an exponent, M_(x) is a mantissa, 1≦M_(x)<2, f_(x) is a N-bit fraction, A_(x) is a value of the most significant K bits of f_(x), B_(x) is a value of the least significant (M−K) bits of f_(x), 0≦K<N, and K, N are natural numbers.

[0025] The present invention is used to compute the logarithm Y. The logarithm Y is given by: $\begin{matrix} \begin{matrix} {Y = {\log_{p}X}} \\ {= {\log_{p}\left\lfloor {\left( \left( {1 + {A_{x} \cdot 2^{- K}} + {B_{x} \cdot 2^{- N}}} \right) \right) \cdot 2^{E_{x}}} \right\rfloor}} \\ {= {{\log_{p}\left( 2^{E_{x}} \right)} + {\log_{p}\left\lbrack {\left( {1 + {A_{x} \cdot 2^{- K}}} \right) \cdot \left( \frac{1 + {A_{x} \cdot 2^{- K}} + {B_{x} \cdot 2^{- N}}}{1 + {A_{x} \cdot 2^{- K}}} \right)} \right\rbrack}}} \\ {= {{E_{x} \cdot {\log_{p}(2)}} + {\log_{p}\left( {1 + {A_{x} \cdot 2^{- K}}} \right)} + {\log_{p}\left( {1 + \frac{B_{x} \cdot 2^{- N}}{1 + {A_{x} \cdot 2^{- K}}}} \right)}}} \\ {= {{E_{x} \cdot {\log_{p}(2)}} + {\log_{p}\left( {1 + {A_{x} \cdot 2^{- K}}} \right)} + {\frac{\ln \left( {1 + R_{d}} \right)}{\ln (p)}\quad {where}}}} \\ {R_{d} = \frac{B_{x} \cdot 2^{- N}}{1 + {A_{x} \cdot 2^{- K}}}} \\ \quad \end{matrix} & (2) \end{matrix}$

[0026] Therefore, to find the logarithm Y, the number whose value is log_(p) 2 and the exponent E_(x) are input to the first multiplier 110 to get the multiplying result R_(m1) to output at first. The value A_(x) is input to the logarithmic table 120 and checks the logarithmic table 120 to get the result of log_(p) (1+A_(x)·2^(−K)) to output. Then, the multiplying result R^(m1) and the result of log_(p) (1+A_(x)·2^(−K)) are added by the first adder 130 to get the adding result R_(d). As well, the value B_(x) is divided by the adding number whose value is (2^(K)+A_(x)) in the divider 140 to get the dividing result R_(d) to output. Then, the dividing result R_(d) is input to the Taylor-Series approximation circuit 150 to find the value of ln(1+R_(d)) to output. The number whose value is 1/ln(p) and the value of ln(1+R_(d)) are input to the second multiplier 170 to get the second multiplying result R_(m2) to output. Finally, the adding result R_(a) and the second multiplying result R_(m2) are input to the second adder 160 to get the logarithm Y to output.

[0027] When computing a logarithm to a base 2 of the floating-point number X, the first multiplier 110 receives a number whose value is log₂ 2 (=1) and the exponent E_(x). Therefore, the first multiplier 110 is not required. When computing a natural logarithm of the floating-point number X, the second multiplier 170 receives a number whose value is 1/ln(e) (=1) and the value of ln(1+R_(d)). Therefore, the second multiplier 170 is not required.

[0028]FIG. 3 is a schematic diagram showing the apparatus for computing a logarithm of a floating-point number according to the present invention. The apparatus 200, as shown in FIG. 3, comprises a multiplier 220, a logarithmic table 220, a first adder 230, a divider 240, a Taylor-Series Approximation Circuit 250, a second adder 260 and a fixed-point circuit 270.

[0029] The apparatus 200 is used for a natural logarithm computation of a floating-point number X. The floating-point number X is an IEEE single precision floating point number, with thirty-two bits: X₃₁, X₃₀, . . . , and X₀. Because the floating-point number X is greater than zero, the sign bit of the floating-point number S_(x) (bit X₃₁) must be zero. Therefore, the floating-point number X is represented in the format of 2^(Ex)·M_(x). The exponent of the floating-point number E_(x) has eight bits: X₃₀, X₂₉, . . . , and X₂₃. The mantissa of the floating-point number M_(x) is represented in the format (1+f_(x)). The fraction of the floating-point number f_(x) has twenty-three bits: X₂₂, X₂₁, . . . , and X₀. The fraction of the floating-point number f_(x) are divided into two parts: A_(x) and B_(x). A_(x) is a value of the most significant 8 bits of f_(x). B_(x) is a value of the least significant 15 bits of f_(x). It means that A_(x) is the value of X₂₂˜X₁₅ and B_(x) is the value of X₁₄˜X₁₀.

[0030] The multiplier 210 receives a number whose value is ln2 and the exponent E_(x) and outputs a multiplying result R_(m1). The logarithmic table 220 receives the value A_(x) and checking the logarithmic table 220 to output a result of ln(1+A_(x)·2⁻⁸). The first adder 230 is connected to the multiplier 210 and the logarithmic table 220 for adding the multiplying result R_(m1) and the result of ln(1+A_(x)·2⁻⁸) to output an adding result R_(a). The divider 240 receives the value B_(x) and an adding number whose value is (2⁸+A_(x)) and divides the value B_(x) by the adding number to output a dividing result R_(d). The Taylor-Series approximation circuit 250 is connected to the divider 240 for receiving the dividing result R_(d), finding a value of ln(1+R_(d)) and outputting the value of ln(1+R_(d)). The second adder 260 is connected to the first adder 230 and the Taylor-Series approximation circuit 250 for adding the adding result R_(a) and the value of ln(1+R_(d)) to output a computed result Y. In this embodiment of the present invention, the apparatus comprises the fixed-point circuit 270. The fixed-point circuit 270 is received the computed result Y and representing the computed result Y in the format of (−1)^(sy)·2^(Ey)·M_(y), where S_(y) is a sign, E_(y) is an exponent, M_(y) is a mantissa, 1≦M_(y)<2.

[0031] The embodiment of the present invention is used to compute the logarithm Y. The logarithm Y is given by: $\begin{matrix} \begin{matrix} {Y = {\ln \quad (X)}} \\ {= {\ln \left\lfloor {\left( \left( {1 + {A_{x} \cdot 2^{- 8}} + {B_{x} \cdot 2^{- 23}}} \right) \right) \cdot 2^{E_{x}}} \right\rfloor}} \\ {= {{\ln \left( 2^{E_{x}} \right)} + {\ln \left\lbrack {\left( {1 + {A_{x} \cdot 2^{- 8}}} \right) \cdot \left( \frac{1 + {A_{x} \cdot 2^{- 8}} + {B_{x} \cdot 2^{- 23}}}{1 + {A_{x} \cdot 2^{- 8}}} \right)} \right\rbrack}}} \\ {= {{E_{x} \cdot {\ln (2)}} + {\ln \left( {1 + {A_{x} \cdot 2^{- 8}}} \right)} + {\ln \left( {1 + \frac{B_{x} \cdot 2^{- 23}}{1 + {A_{x} \cdot 2^{- 8}}}} \right)}}} \\ {= {{E_{x} \cdot {\ln (2)}} + {\ln \left( {1 + {A_{x} \cdot 2^{- 8}}} \right)} + {{\ln \left( {1 + R_{d}} \right)}\quad {where}}}} \\ {R_{d} = \frac{B_{x} \cdot 2^{- 23}}{1 + {A_{x} \cdot 2^{- 8}}}} \\ \quad \end{matrix} & (3) \end{matrix}$

[0032] Therefore, to find the logarithm Y, a number whose value is ln2 and the exponent E_(x) are input to the multiplier 210 to get the multiplying result R_(m1) to output at first. The value A_(x) is input to the logarithmic table 220 and check the logarithmic table 220 to get the result of ln(1+A_(x)·2⁻⁸) to output. Then, the multiplying result R_(m1) and the result of ln(1+A_(x)·2⁻⁸) are added by the first adder 230 to get the adding result R_(a). As well, the value B_(x) is divided by the adding number whose value is (2⁸+A_(x)) in the divider 240 to get the dividing result R_(d) to output. Then, the dividing result R_(d) is input to the Taylor-Series approximation circuit 250 to find the value of ln(1+R_(d)) to output (referring FIG. 4). Finally, the adding result R_(a) and the value of ln(1+R_(d)) are input to the second adder 260 to get the logarithm Y to output.

[0033]FIG. 4 is a schematic diagram showing one example of the apparatus for computing a Taylor-Series approximation circuit according to FIG. 3. The Taylor-Series approximation circuit 250 is a predetermined circuit to find the value of ln(1+R_(d)) with three-term approximation. As shown in FIG. 5, the Taylor-Series approximation circuit 250 comprises three multiplier 252 a˜252 c, a device for subtraction 254 and a adder 256.

[0034] The value of ln(1+R_(d)) is approximated as: $\begin{matrix} {{\ln \left( {1 + R_{d}} \right)} \approx {R_{d} - \frac{R_{d}^{2}}{2} + \frac{R_{d}^{3}}{3}}} & (4) \end{matrix}$

[0035] Two of the dividing results R_(d) are input to the multiplier 252 a. The multiplier 252 a outputs a value of R_(d) ². Then, the value of R_(d) ² is shifted 1-bit right to obtain a value of R_(d) ²/2. One dividing result R_(d) and the value of R_(d) ²/2 are input into the device for subtraction 254. As well, one dividing result R_(d) and a number whose value is {fraction (1/3)} are input to the multiplier 252 b. The multiplier 252 b outputs a value of R_(d)/3. The value of R_(d)/3 and the value of R_(d) ² are input to the multiplier 252 c. The multiplier 252 c outputs a value of R_(d) ³/3. The adder 256 is connected to the subtraction 254 and the device for subtraction 254 to output the value of ln(1+R_(d))

[0036]FIG. 5 is a schematic diagram showing the apparatus for computing a logarithm of a floating-point number according to another embodiment of the present invention. The apparatus 300, as shown in FIG. 5, comprises a logarithmic table 320, a first adder 330, a divider 340, a Taylor-Series Approximation Circuit 350, a second adder 360 and a multiplier 370.

[0037] The apparatus 300 is used to compute logarithm base-2 of a floating-point number X. The floating-point number X is an IEEE single precision floating point number, with thirty-two bits: X₃₁, X₃₀, . . . , and X₀. Because the floating-point number X is greater than zero, the sign bit of the floating-point number S_(x) (bit X₃₁) must be zero. Therefore, the floating-point number X is represented in the format of 2^(Ex)·M_(x). The exponent of the floating-point number E_(x) has eight bits: X₃₀, X₂₉, . . . , and X₂₃. The mantissa of the floating-point number M_(x) is represented in the format (1+f_(x)). The fraction of the floating-point number f_(x) has twenty-three bits: X₂₂, X₂₁, . . . , and X₀. The fraction of the floating-point number f_(x)are divided into two parts: A_(x) and B_(x). A_(x) is a value of the most significant 8 bits of f_(x). B_(x) is a value of the least significant 15 bits of f_(x). It means that A_(x) is the value of X₂₂˜X₁₅ and B_(x) is the value of X₁₄˜X₁₀.

[0038] The logarithmic table 320 receives the value A_(x) and checking the logarithmic table 320 to output a result of log₂ (1+A_(x)·2⁻⁸) The first adder 330 receives the exponent E_(x) and the result of log₂(1+A_(x)·2⁻⁸) to output an adding result R_(a). The divider 340 receives the value B_(x) and an adding number whose value is (2⁸+A_(x)) and divides the value B_(x) by the adding number to output a dividing result R_(d). The Taylor-Series approximation circuit 350 is connected to the divider 340 for receiving the dividing result R_(d), finding a value of ln(1+R_(d)) and outputting the value of ln(1+R_(d)). The multiplier 370 receives a number whose value is 1/ln(2) and the value of ln(1+R_(d)) to output a second multiplying result R_(m2). The second adder 360 connected to the first adder 350 and the multiplier 370 for adding the adding result R_(a) and the second multiplying result R_(m2) to output the logarithm Y.

[0039] The embodiment is used to compute logarithm base-2 of the floating-point number X. The computed result Y is given by: $\begin{matrix} \begin{matrix} {Y = {\log_{2}(X)}} \\ {= {\log_{2}\left\lfloor {\left( \left( {1 + {A_{x} \cdot 2^{- 8}} + {B_{x} \cdot 2^{- 23}}} \right) \right) \cdot 2^{E_{x}}} \right\rfloor}} \\ {= {{\log_{2}\left( 2^{E_{x}} \right)} + {\log_{2}\left\lbrack {\left( {1 + {A_{x} \cdot 2^{- 8}}} \right) \cdot \left( \frac{1 + {A_{x} \cdot 2^{- 8}} + {B_{x} \cdot 2^{- 23}}}{1 + {A_{x} \cdot 2^{- 8}}} \right)} \right\rbrack}}} \\ {= {{E_{x} \cdot {\log_{2}(2)}} + {\log_{2}\left( {1 + {A_{x} \cdot 2^{- 8}}} \right)} + {\log_{2}\left( {1 + \frac{B_{x} \cdot 2^{- 23}}{1 + {A_{x} \cdot 2^{- 8}}}} \right)}}} \\ {= {E_{x} + {\log_{2}\left( {1 + {A_{x} \cdot 2^{- 8}}} \right)} + {\frac{\ln \left( {1 + R_{d}} \right)}{\ln (2)}\quad {where}}}} \\ {R_{d} = \frac{B_{x} \cdot 2^{- 23}}{1 + {A_{x} \cdot 2^{- 8}}}} \\ \quad \end{matrix} & (4) \end{matrix}$

[0040] Therefore, to find the logarithm Y, the value A_(x) is input to the logarithmic table 320 and checks the logarithmic table 320 to get the result of ln(1+A_(x)·2⁻⁸) to output. Then, the exponent E_(x) and the result of ln(1+A_(x)·2⁻⁸) are added by the first adder 330 to get the adding result R_(a). As well, the value B_(x) is divided by the adding number whose value is (2⁸+A_(x)) in the divider 340 to get the dividing result R_(d) to output. Then, the dividing result R_(d) is input to the Taylor-Series approximation circuit 350 to find the value of ln(1+R_(d)) to output. A number whose value is 1/ln(2) and the value of ln(1+R_(d)) are input to the multiplier 370 to get a second multiplying result R_(m2) output. Finally, the adding result R_(a) and the second multiplying result R_(m2) are input to the second adder 360 to get the logarithm Y to output.

[0041] While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. An apparatus for computing a logarithm to a base p of a floating-point number X wherein the floating-point number X is represented in the format of (−1)^(Sx)·2^(Ex)·M_(x), where M_(x) =(1+f_(x))=(1+A_(x)·2^(−K))+(B_(x)·2^(−N)), where S_(x) is a sign, E_(x) is an exponent, M_(x) is a mantissa, 1≦M_(x)<2, f_(x) is a N-bit fraction, A_(x) is a value of the most significant K bits of f_(x), B_(x) is a value of the least significant (N−K) bits of f_(x), 0≦K<N, and p, K, N are natural numbers, the apparatus comprising: a first multiplier for multiplying a number whose value is log_(p) 2 and the exponent E_(x) and outputting a multiplying result; a logarithmic table for receiving the value A_(x) and checking the logarithmic table to output a result; a first adder connected to the first multiplier and the logarithmic table for adding the multiplying result and the result to output an adding result; a divider for receiving the value B_(x) and an adding number whose value is (2^(K)+A_(x)) and dividing the value B_(x) by the adding number to output a dividing result R_(d); a Taylor-Series approximation circuit connected to the divider for receiving the dividing result R_(d), finding a value of ln(1+R_(d)) and outputting the value of ln(1+R_(d)); a second multiplier for multiplying a number whose value is 1/ln(p) and the value of ln(1+R_(d)) to output a second multiplying result; and a second adder connected to the first adder and the second multiplier for adding the adding result and the second multiplying result to output the logarithm Y.
 2. The apparatus as claimed in claim 1, wherein, when computing a logarithm to a base 2 of a floating-point number X, the first multiplier is not required.
 3. The apparatus as claimed in claim 1, wherein, when computing a natural logarithm of a floating-point number X, the second multiplier is not required.
 4. The apparatus as claimed in claim 1 further comprising a fixed-point circuit for receiving the logarithm Y and representing the logarithm Y in the format of (−1)^(Sy)·2^(Ey)·M_(y), where S_(y) is a sign, E_(y) is an exponent, M_(y) is a mantissa, 1≦M_(y)<2.
 5. The apparatus as claimed in claim 1, wherein the logarithmic table is a predetermined logarithmic table for computing the logarithm of the value A_(x) to the base p.
 6. The apparatus as claimed in claim 1, wherein the Taylor-Series approximation circuit is a predetermined circuit to find the value of ln(1+R_(d)) with three-term approximation.
 7. A method for computing a logarithm to a base p of a floating-point number X wherein the floating-point number X is represented in the format of (−1)^(Sx)·2^(Ex)·M_(x), where M_(x)=(1+f_(x))=(1+A_(x)·2^(−K))+(B_(x)·2^(−N)), where S_(x) is a sign, E_(x) is an exponent, M_(x) is a mantissa, 1≦M_(x)<2, f_(x) is a N-bit fraction, A_(x) is a value of the most significant K bits of f_(x), B_(x) is a value of the least significant (N−K) bits of f_(x), 0≦K<N, and p, K, N are natural numbers, the method comprising the steps of: multiplying a number whose value is log_(p 2) and the exponent E_(x) to get a multiplying result to output by a first multiplier; receiving the value A_(x) by a logarithmic table and checking the logarithmic table to get a result to output; adding the multiplying result and the result to get an adding result to output by a first adder; dividing the value B_(x) by an adding number whose value is (2^(K)+A_(x)) to get a dividing result R_(d) to output by a divider; receiving the dividing result R_(d) and finding a value of ln(1+R_(d)) to output by a Taylor-Series approximation circuit; multiplying a number whose value is 1/ln(p) and the value of ln(1+R_(d)) to get a second multiplying result to output by a second multiplier; and adding the adding result and the second multiplying result to get the logarithm Y to output by a second adder.
 8. The method as claimed in claim 7, wherein, when computing a logarithm to a base 2 of a floating-point number X, the first multiplier is not required.
 9. The method as claimed in claim 7, wherein, when computing a natural logarithm of a floating-point number X, the second multiplier is not required.
 10. The method as claimed in claim 7 further comprising a fixed-point circuit for receiving the logarithm Y and representing the logarithm Y in the format of (−1)^(Sy)·2^(Ey ·M) _(y), where S_(x) is a sign, E_(x) is an exponent, M_(x) is a mantissa, 1≦M_(x)<2.
 11. The method as claimed in claim 7, wherein the logarithmic table is a predetermined logarithmic table for computing the logarithm of the value A_(x) to the base p.
 12. The method as claimed in claim 4, wherein the Taylor-Series approximation circuit is a predetermined circuit to find the value of ln(1+R_(d)) with three-term approximation. 